Solid propellant burning rate detector



,.1965 J. E. FITZGERALD ETAL 3,201,973

SOLID PRQPELLANT BURNING RATE DETECTOR Filed Dec. 14, 1962 2 Sheets-Sheet 1 INVENTORS. J. EDMUND FITZGERALD NORMAN C. ALLEN BY ATTORN EY.

1965 J. E. FITZGERALD ETAL 3,201,973

SOLID PROPELLANT BURNING RATE DETECTOR 2 Sheets-Sheet 2 Filed Dec. 14, 1962 2: OMZEDQ IFOZMJ Z N O fi.z QmZmDm IPOZMJ Z N O 5 4 3 2 I 0 J J J C E S E S N I N 4 w 0 N m 3 m 3. o. B O C E m E R H R R E S S T U S G S N N S N l E E W l L R 7 9 P 4 3 7 2 8 4 R a o 4 o M O M R N R M G C l w F R m 9 7 2 C. O E s a R w P l N Q M W 5 2 3 4 5 E l l S W 0 O O N B O N N N N G N l L E M R u E M E M H N N N N l C N N N N T A A A A H H H H 0 O 0 0 0 0 c C C C 0 O O O O O 8 6 4 2 IZwnS mKDwwwmQ Emmi/IO INVENTORS. J. EDMUND FITZGERALD NORMAN C. ALLEN BY TIME (SECJ FIG. 5.

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United States Patent O" 3,201,973 SOLID PRDPELLANT BURNING RATE DETECTOR John E. Fitzgerald and Norman C. Allen, Redlands, Califi, assiguors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Dec. 14, 1962, Ser. No. 244,849 Claims. (Cl. 73-35) The present invention relates to a light transmitting apparatus and more particularly to instrumentation and a method for accurately measuring the burning rate of solid propellant rocket motors.

The burning rate is one of the most important properties of a composition designed for use as a propellant. Its measurement is an experiment of fundamental importance, both as a preliminary to the designing of propellant charges and in the study of the mechanism of the burning reaction. Burning rates have usually been determined by firing the powder charge in a closed vessel or in a vented chamber simulating a rocket motor. The variation of chamber pressure with time is observed. Such methods have not been entirely satisfactory because certain questionable assumptions are always involved in utilizing the pressure data to calculate the linear regression of the burning surface; the precision is generally too poor to show clearly slight ditferences between propellants, and the apparatus is complicated, expensive and requires elaborate safety precautions.

The general purpose of the present invention is to provide simple instrumentation which will accurately measure the burning rate of solid propellant rocket motors.

An object of. the present invention is the provision of a burning rate detector which will have no effect on propellant burning characteristics.

Another object is to provide a burning rate detector method which will require no major modifications of existing motor chambers. 1

A further object of the invention is the provision of a light energy transmitting apparatus which is simple to construct and is reliable.

Still another object is to provide a device having the necessary resolution to detect changes in propellant burning rate.

The objects of the invention are accomplished by use of light-transmitting rods of different lengths embedded in a propellant grain and transmitting light energy during burning of the grain to light-sensitive elements which generate signals indicative of the rate of grain burning.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of the invention with parts in section;

FIG. 2 is a sectional view showing the relation of the invention to a solid propellant charge;

FIG. 3 is a sectional plan view generally along line 3-3 of FIG. 1;

FIG. 4 is a graph showing pressure and grain length burned versus time; and

FIG. 5 illustrates the general appearance of oscillograph traces of the recording channels.

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown schematically in FIG. 1 an instrumentation arrangement of the present invention designated generally by refenerce numeral 10. The arrangement comprises a light transmitting apparatus in the form of an array of Lucite rods 11 arranged in parallel relation. It is to be noted that the rods are of unequal 3,2hl,9.73 Patented Aug. 24, 1965 length and are coated with an opaque material for a purpose later made apparent. Lucite is a tradenarne applied to a transparent synthetic resin, polymerized methyl methacrylate, which is outstanding for its clarity and transparency and is used herein for the light energy transmitting rods.

Rods 11 are supported on a plate 12, of stainless steel or the like, and are held thereon by a pair of angle members 13, 13' suitably bonded to the rods and the plate, as by epoxy resin or the like. Plate 12 is provided with perforations 14 so arranged that there is a perforation adjacent the facing end of each rod 11, respectively, the dimensions of the perforation being slightly smaller than the cross-section of the respective rod. Adjacent each perforation or opening 14, respectively, is a light sensitive device 15, for example a silicon photocell which generates a direct current signal of known polarity upon exposure to light energy on the visible and near-infrared portion of the spectrum. Devices 15 are housed in a recess of depression 16 formed in a supporting member 17. Devices 15 are provided with suitable electrical leads 18 encased in a tube or cable housing 19 for connection, through a junction box 20 and amplifiers 21 to suitable recording equipment 22 such, for example, as a recording oscillograph high frequency galvanometer, providing a flat frequency response to 3000 c.p.s. Turning now to FIG. 2, the light transmitting apparatus is shown applied to a solid propellant rocket motor designated generally by reference numeral 23 and comprising a casing 24 having an inner liner 25 of fibreglass or the like, in which a propellant grain or charge 26 is cast. Support member 17 is bonded or otherwise secured to the bottom of casing 24. Leads 18 are formed into a cable which is passed through tube 27, which is locked in place by nut 28 and a packing gland 29.

In operation, the light transmitting apparatus is installed in a rocket motor casing after the manner shown in FIG. 2 and the propellant grain or charge 26 is cast therearound. After curing the grain, the test motor is mounted on a test stand and leads 18 are suitably connected to recorder 22. The grain is then ignited at the face of the grain by any suitable ignition means, for example, a lamination of silk impregnated with an igniter composition and initiated by an embedded hot wire. As the flame front progresses through the grain, the end faces of the different length Lucite rods ll are exposed in succession, light energy is transmitted by the rods to the respective photocells or devices 15 and signals are generated. The resulting output from each cell drives a recording channel of the recorder 22 and is indicative of the rate of grain burning see FIG. 5. The opaque coating on rods renders each recording channel optically and electrically independent and prevents light cross-talk between the channels.

The propellant burning rate is defined as the ratio of distance between the sensing Lucite rod ends (ds) to time between photocell signals (dt), as taken from the oscillograph record. FIG. 4 is a graph showing pressure and grain length burned versus time. Average burning rate is the average over the entire incremental span, and maximum burning rate is the increment with the highest rate. Measurement resolution is limited only by the oscillograph chart speed. Response of the oscillograph system is approximately 50 microseconds when responding to a pulsed signal and is adequate for measurement of the transition from normal to high burning rates. Use of a tape recorder could improve system response to about 10 microsecond rise time.

T he principal advantage of this measurement system is that it records both the propellant burning rate and the ticular value and interest. This property enables the cell to sense shock waves propagating through the propellant grain, thus performing a dual function. These shock disturbances in the grain characteristically precede extreme burning rates that lead to motor destruction.

' Extensive testing was done on various types solid propellant grains. It was found that Nitrasol propellant had a detrimental effect on Lucite and on the paint used to opacify the sides of the rods, causing light cross-talk between recording channels. However, the signal level was sufficient to discriminate between channels. This problem did not arise with polyurethane propellants.

' There has thus been provided means for accurately measuring the burning rate of propellants which is simple, economical and reliable.

While the embodiment of the invention described shows the array of rods linearly aligned, it is understood the rods could be arrayed in other configurations, for example, a cluster in circular form, or spaced in the propellant grain at known intervals without departing from the invention.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A solid propellant burning rate detector comprising in combination:

a rocket motor;

a propellant grain within said motor;

an array of light energy transmitting rods of unequal length linearly aligned within said grain; and

an array of photovoltaic cells arranged opposite the base of each rod and adapted to be connected to a recording means; whereby light energy created during the burning of said grain and transmitted through said rods amplifies the output of the cells creating a pulse signal which is recorded by said means.

2. A solid propellant burning rate detector device comprising in combination:

a rocket motor casing;

a propellant grain secured within said motor casing;

a plurality of Lucite rods of unequal lengths arranged in parallel relation within said grain; and

a plurality of sensing cells, each of which is arranged adjacent the base of a single Lucite rod, said cells adapted to be connected to a recording system whereby light energy transmitted through the successively exposed rods amplifies the cell output creating a pulse signal which is recorded.

3. A solid propellant burning rate measuring device comprising in combination:

.a rocket motor casing;

a solid propellant grain secured within said casin g;

six light transmitting rods of unequal length arranged in parallel relation to each other within said grain; 81X photovoltaic cells mounted so that each cell is posit oned opposite the end face of a single rod forming SIX separate measuring channels; and

a recorder connected to said cells by electrical means; whereby as the burning progresses through the grain, the end faces of the difierent length rods are exposed in succession and light energy created by such burning is transmitted by the rods to the respective cells where signals are generated which are recorded.

4. Claim 3 wherein the sides of said rods are coated with an opaque material to prevent interaction between said measuring channels.

5. A sol-id propellant burning ra-te instrumentation comprising in combination:

a rocket motor casing;

.a solid propellant grain in bonded relation to said casing having been cast and cured therein;

a burning rate instrumentation mounted to the bottom of said casing and extending into said grain in a bonded relation thereto, the grain having been cast around said instrumentation within said casing; and

a recording device connected to said instrumentation by electrical means;

said instrumentation comprising a support plate provided with a plurality of perforations in parallel relation to each other;

a plurality of light transmitting rods of unequal length r bonded to said plate and so arranged that a perforatron is adjacent the facing end of each rod; and

a plurality of light sensitive cells secured to said plate each cell being mounted adjacent a single perforatiori and opposite said facing end of a single rod thereby forming a single measuring channel.

References Cited by the Examiner UNITED STATES PATENTS 9/41 Metcalf 2s0 227 5/61 Vollmer 73--290 X 2/64 Vasel -Q 881 OTHER REFERENCES RICHARD C. QUEISSER, Primary Examiner. DAVID SCHONBERG, Examiner. 

1. A SOLID PROPELLANT BURNING RATE DETECTOR COMPRISING IN COMBINATION: A ROCKET MOTOR; A PROPELLANT GRAIN WITHIN SAID MOTOR; AN ARRAY OF LIGHT ENERGY TRANSMITTING RODS OF UNEQUAL LENGTH LINEARLY ALINGNED WITHIN SAID GRAIN; AND AN ARRAY OF PHOTOVOLTAIC CELLS ARRANGED OPPOSITE THE BASE OF EACH ROD AND ADAPTED TO BE CONNECTED TO A RECORDING MEANS; WHEREBY LIGHT ENERGY CREATED DUR- 